Late-Pleistocene slope evolution using characteristics of rock cities in Western New York, USA

Exposed massive bedrock cliffs often exhibiting a warren-like network of Joints and crevices are common features in resistant conglomerates and sandstones of the Allegheny Plateau in New York, Pennsylvania and Ohio. Located south of the former extent of glacial ice, they are locally referred to as rock cities. In this study, a detailed mapping campaign of Thunder Rocks, an exposure of Olean Conglomerate of the Pennsylvanian Pottsville group, was conducted. Rock hardness data using a Schmidt hammer were collected to test the hypothesis that slow, downslope movement under periglacial conditions was responsible for the presence and distribution of detached blocks. Slope data were collected at the site and assessed from a 10 m Digital Elevation Model (DEM) in ArcGIS. Elevation and aspect were also extracted from the DEM for analyses. The distribution of rocks indicates a downslope decrease in dimension (Pearson r = 0.713, α = 0.01). Hardness values measured normal to the bedding show a decrease downslope (r = 0.678; α = 0.01); rock hardness values measured parallel to the bedding do not show this relation. The spatial distribution of hardness values indicates two distinct periods of downslope transfer. The presence of two colluvial units in the region supports this observation and suggests that the timing of block movement is associated with slope instability following maximum glacial conditions, not during the period of most intense periglacial activity.